Significantly improved peak rates of 1 Gbps in the downlink and 500 Mbps in the uplink are required for a Long Term Evolution-Advanced (LTE-A) system as compared to a Long Term Evolution (LTE) system. Also good compatibility of the LTE-A system with the LTE system is required. Carrier Aggregation (CA) is introduced to the LTE-A system to accommodate the requirement on improved peak rates, compatibility with the LTE system and full use of spectrum resources.
Carrier aggregation refers to a mechanism in which a User Equipment (UE) can aggregate a plurality of cells concurrently and the plurality of cells can provide the UE concurrently with a data transmission service. In the system with carrier aggregation, component carriers corresponding to the respective cells may be consecutive or inconsecutive in the frequency domain, the maximum bandwidth of each component carrier is 20 MHz for compatibility with the LTE system, and there is a bandwidth which may be the same or different across the respective component carriers.
A random access in the LTE system happens generally for the following several reasons:
In a first scenario, there is an access to the system from a Radio Resource Control Idle (RRC_IDLE) status (also referred to an initial access);
In a second scenario, a Radio Resource Control (RRC) connection reestablishment is initiated after a radio link fails (also deemed as an initial access);
In a third scenario, a random access is required during a handover;
In a fourth scenario, a UE in a Radio Resource Control Connected (RRC_CONNECTED) status has downlink data arrival; and
In a fifth scenario, a UE in an RRC_CONNECTED status has uplink data arrival.
In the third and fourth scenarios, if there is a dedicated preamble, then a contention-free random access can be performed, and FIG. 1 illustrates a contention-free random access procedure which generally includes the following three steps:
For a message 0 (Msg0): a base station assigns a UE with a dedicated random access preamble (Random Access Preamble, ra-PreambleIndex) for a contention-free random access and the mask index of a Physical Random Access Channel (PRACH) (ra-PRACH-MaskIndex) for the random access. For a contention-free random access due to incoming downlink data, such information is carried over a Physical Downlink Control Channel (PDCCH), and for a contention-free random access due to a handover, such information is carried in a handover command;
For a message 1 (Msg1): the UE sends the specified dedicated preamble to the base station over the specified PRACH resource according to the ra-PreambleIndex and the ra-PRACH-MaskIndex indicated by the Msg0. The base station calculates an uplink Timing Advance (TA) from the Msg1 upon reception of the Msg1; and
For a Message 2 (Msg2): the base station sends a random access response including information on the timing advance to the UE to notify the UE of the timing advance for uplink transmission until acquisition of a next TA command.
A contention-based random access can be adopted for a random access due to any of the other random access reasons, and FIG. 2 illustrates a contention-based random access procedure which generally includes the following four steps:
For a message 1 (Msg1): a UE selects a random access preamble and a PRACH resource and sends the selected random access preamble to a base station over the PRACH resource;
For a message 2 (Msg2): the base station receives the preamble, calculates a TA and sends to the UE a random access response including at least information on the timing advance and uplink scheduling signaling (UL grant) for a message 3 (Msg3);
For the message 3 (Msg3): the UE performs uplink transmission over a resource specified by the UL grant in the Msg2, and contents of the uplink transmission vary from one random access reason to another, for example, an RRC connection establishment request is sent in the Msg3 for an initial access; and
For a message 4 (Msg4): the base station sends a contention resolution message to the UE, and the UE can judge from the Msg4 whether the random access succeeds.
An uplink synchronization procedure is for the purpose of keeping the UE and the base station in uplink synchronization so that the UE sends uplink data and sends feedback information of a Hybrid Automatic Repeat Request (HARM) for downlink data.
Uplink synchronization is maintained by the base station as stipulated in the LTE system. For a random access of the UE, the base station acquires the initial timing advance from the preamble, and a subsequent uplink synchronization maintenance procedure is as illustrated in FIG. 3:
The base station and the UE maintain an uplink synchronization timer (TA Timer or TAT) respectively; the base station sends a TA command (TA cmd) to the UE and starts the TAT; if the UE can not receive the TA command correctly, then it sends a Negative Acknowledgement (NACK) message to the base station, and the base station receives the NACK, resends a TA command in an appropriate sub-frame and restarts the TAT; or if the UE receives the TA command correctly, then it starts the TAT of the UE and sends an Acknowledgement (ACK) message to the base station, and the base station restarts the TAT of the base station upon reception of the ACK message sent from the UE. The base station considers a specific UE as being synchronized if its TAT for the UE does not expire; and the UE also considers itself as being synchronized as long as the TAT maintained by the UE itself does not expire.
Two scenarios in support of multi-TA are currently defined in the 3rd Generation Partnership Project (3GPP).
The first scenario is a scenario in which a Remote Radio Head (RRH) is introduced.
As illustrated in FIG. 4, a large coverage area is provided at a frequency F1 (i.e., an F1 cell), a remote head is used at a frequency F2 for hotspot coverage in the F1 cell (i.e., an F2 cell), and mobility management is performed based upon the F1. In this scenario, if a UE is located in an area where the F2 RRH cell and the F1 cell overlap, then the F1 cell and the F2 cell can be aggregated, but there are different uplink TAs (UL TAs) for the F1 cell and the F2 cell.
The second scenario is a scenario in which a repeater is introduced.
As illustrated in FIG. 5, a base station supports the F1 with a large coverage area and the F2 with a small coverage area, and the coverage area of the F2 can be extended by a frequency selective repeater. In this scenario, if a UE is located in an area where the F1 cell and the F2 cell overlap, then the F1 cell and the F2 cell can be aggregated, but there are different UL TAs for the F1 cell and the F2 cell.
In order to facilitate maintenance of the TAs in the multi-TA system, the concept of TA group is introduced, and the same TA can be used for Uplink Component Carriers (UL CCs) of cells belonging to the same TA group, and different TAs can be used for UL CCs of cells belonging to different TA groups.
The inventors have identified during making of the invention the following technical problem in the prior art:
In the system with carrier aggregation, the UE performs uplink transmission in different cells possibly with the use of different TAs, but there has been absent so far a specific solution for how the UE acquires a TA for uplink transmission in a newly configured cell.